Battery operated devices such as mobile telephones, portable computers, calculators, cameras, Personal Digital Assistants (PDAs), video game controllers, etc. typically include DC-DC converters to produce a constant power supply voltage at the load. Although a constant voltage is provided at the load, the battery voltage decreases as the battery is discharged. These circuits include switches that alternate connecting an inductor between the battery and a load and between the load and ground using low-loss switches, which are usually operated at a fixed frequency. In other words, the circuits switch the connections such that during one part of the clock period the inductor is connected between the battery and a load and during another part of the clock period the switches connect the inductor between the load and ground. Alternatively, they can alternate connecting an inductor between the battery and ground and between the load and ground. The load is shunted by a large capacitor which absorbs the Alternating Current (“AC”) components leaving a load voltage with low ripple.
DC-DC converters include a negative feedback loop which matches a portion of the load voltage to a reference voltage by modifying the duty cycle of the switches. Stabilizing the negative feedback loop is difficult because it includes an inductor and shunt capacitor and operates with a wide range of load currents. One technique for stabilizing the feedback loop includes designing the loop to have two series connected comparators, wherein the first comparator produces a current output in response to the load voltage error and the second comparator controls the duty cycle of the switches to adapt the peak inductor current to the output of the first comparator. This technique is called current mode control or current programmed control. During the intervals in which the inductor is connected across the load its current may reverse if the load current is too low. To prevent this from occurring a third comparator may be included in the feedback loop.
Contemporary circuits typically use Complementary Metal Oxide Semiconductor (CMOS) technology to manufacture the comparators. This technology provides comparators having low loss switches and that can be put into low power standby modes. However, they use many high valued resistors to limit the current of the controller circuits. In addition, manufacturing high valued resistors using CMOS technology consumes large areas of the semiconductor material. The use of large areas also introduces large stray capacitances, which are undesirable because they lower the switching speeds and increase power consumption of the CMOS devices.
Hence, a need exists for a DC-DC converter and a method of compensating for offset errors in a DC-DC converter. It would be advantageous for the DC-DC converter to be cost and time efficient to manufacture.